The new “internet” economy has created a need for better appearing and for higher performance papers. Desirable aesthetic qualities require papers to be brighter, more opaque, and to have a smoother printing surface. Better qualities of these properties are useful to accommodate increasingly important uses of papers, such as ink jet printing and desktop publishing. And, higher performance requirements are sought in industrial papers like paper board and packaging (used to ship items ordered via the internet) which paper types require components with very high mechanical strength. At the same time, escalating shipping and postal costs mean that users would also benefit from lighter weight papers. In other words, it would be desirable to provide industrial paper with higher mechanical strength, higher brightness, higher opacity, yet still having improved printability, weigh less, and made at a lower basis weight. With paper fillers and paper manufacturing techniques known or practiced heretofore, the juxtaposition of these requirements has not been feasible.
Presently, the paper industry adds filler to paper in order to improve some of the key performance attributes. However, nearly all of the paper fillers currently available have some drawbacks associated with their use. For example, the best known filler for improving the optical properties of paper is titanium dioxide (TiO2). Although it can be used to dramatically improve the optical properties of paper, unfortunately, TiO2 has some major disadvantages. First, it is very expensive, on the order of about US$2,000 per ton at time of filing of this patent application. Second, it is highly abrasive to processing machinery. Third, it is usually shipped in a slurry form, which requires dispersant additives to prevent settling; that both adds cost and creates paper machine runnability issues. Finally, TiO2 must be produced off-site, remotely from the paper mill, shipped to customers, thus adding a large transportation cost.
Other fillers, like silica and calcium silicate, are sometimes used to improve sheet bulk, print quality, and other paper properties. Here again, one of the major disadvantages to these products is that they reduce sheet strength and are expensive (silica at about US$3,000 to about US$4,000 per ton, and calcium silicate at about US$800 to about US$1,000 per ton at time of filing this patent application). These products, like TiO2, are also typically produced off-site, and carry a large transportation cost.
In an effort to reduce production costs, the industry is increasingly switching to commodity fillers such as ground or precipitated calcium carbonate. Precipitated calcium carbonate is perhaps best suited for improving the optical and print qualities of paper. It can also be produced on-site (at a paper mill), thus reducing or eliminating transportation costs. Unfortunately, calcium carbonate is not as effective as TiO2 or silica based fillers. Moreover, it significantly reduces the strength performance of paper, in addition to causing undesirable dusting.
Importantly, it must also be appreciated that the conventional process for the manufacture of PCC also has several limitations. First, the reaction takes place at atmospheric pressure in an “open” vessel, using a batch process technique. Such processes also require a high CO2 concentration (15%-20% by volume) in a combustion gas waste stream, and require a long reaction time (often from about 180 to about 200 minutes) to achieve the formation of the desired carbonate species. Conventional PCC manufacturing also requires large size reactors, (about 200 USgal/ton/day of PCC capacity). Additionally, a large building is required to house such PCC reactors, and consequently, a large sized site is a requirement for such a reactor building. Resultantly, capital for the building, equipment, and construction is significant.
In view of the above, there is a definite and as yet unmet need in the paper industry for a high performance specialty filler manufacturing process, especially for such a process that can produce important commodity fillers like PCC. Moreover, it would be desirable for such a high performance process to be technologically superior, i.e., produce better quality filler products at lower costs than are presently available.
Also, it would be desirable to find a new, high performance specialty filler that would, among other things, improve the aesthetic properties of paper (brightness, opacity, smoothness, print quality, etc.) as well as, and at the same time, and the mechanical properties of paper, (bulk, stiffness, etc.), without decreasing any strength properties. Even more desirably, such a filler would be available for supply as a slurry that is free of dispersant. Finally, it would be desirable that such a high performance filler be available from an on-site production facility, in order to eliminate transportation costs for the finished filler.
With regard to the production of precipitated calcium carbonate, it would also be desirable to provide improvements over conventional batch process so as to reduce or eliminate certain current limitations. For example, in an ideal situation, it would be desirable to produce precipitated calcium carbonate using very low concentration CO2 (as low as about 5.0% CO2 by volume, or less), while nonetheless significantly increasing the reaction rate, in order to reduce the size of reactors required from the current 200 gal/ton/day by at least half, and more preferably, by at least one fourth, i.e, to as low as about 50 gal/ton/day or less reactor size. Moreover, it would be desirable to reduce required building size, and thus reduce both process equipment costs and overall capital costs for such plants.
Finally, it would be highly desirable to develop a new method and apparatus that, with common capital equipment, was capable of both the production of the above mentioned high performance specialty filler as well as the production of precipitated calcium carbonate.